Generally, a slide rail assembly includes at least two rails, which can be moved with respect to each other. To ensure smooth rail movement, it is sometimes required that a sliding assistance device be movably mounted between the rails, wherein the sliding assistance device typically includes bearing balls. For instance, U.S. Pat. No. 7,213,896 B2 discloses a flexible bearing spacer having a plurality of apertures with bearing balls respectively received therein to facilitate relative displacement of two rails. The bearing spacer further has at least one notch in order to be compressible.